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| Home > Publications database > Entwicklung einer In-situ-Messmethode zur Bestimmung des Wasserstoffgehalts amorpher Kohlenwasserstoffschichten in Fusionsanlagen |
| Dissertation / PhD Thesis/Book | PreJuSER-1309 |
2008
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/3621
Report No.: Juel-4279
Abstract: The use of carbon for plasma-facing components in ITER is assumed to cause the safety limit of a maximum of 350 g tritium in the reactor vessel to be exceeded. Tritium is trapped primarily in amorphous hydrocarbon layers on the reactor vessel's surfaces. Within this paper a diagnostic method, the laser-induced thermal desorption spectroscopy (LDS), will be developed to measure in situ the hydrogen inventory in the surface of plasma-facing components in fusion experiments. Its capabilities will be demonstrated in TEXTOR. In LDS, during the plasma discharge a laser beam is used to heat a spot on a surface close to the plasma to a temperature of 1400 to 2100 K to a depth of 100 μm. Trapped hydrogen will be released into the plasma where it emits line radiation. The emitted H$_{α}$-light is quantitatively measured. The amount of released hydrogen is calculated from the intensity of this emission using conversion factors (S/XB)$_{eff}$. The laser light (Nd:YAG, 1064 nm) is conducted via light fibres. At TEXTOR, a 5 mm$^{2}$ sized homogeneous laser spot is created with a pulse duration of 1.5 ms, and an Energy of 5 J, typically. Below the laser spot a volume of at most 1 mm$^{3}$ is desorbed. The generated temperature is calculated numerically and indirectly deduced from surface changings. Depending on the conditions during the layer formation the hydrogen content of the hydrocarbon layer will vary and different fractions of the released molecules (H$_{2}, CH_{4}, C_{2}H_{4}$) are created during the laser heating. Hydrogen-rich layers release a greater fraction of hydrocarbons whereas graphite exposed in the erosion zone releases principally hydrogen molecules. The release of atomic hydrogen by laser desorption was not found. The emitted light is measured by means of narrow-band interference filters and a CCD-camera. The fraction of the light emission which lies outside the observation volume is estimated using simulations of the emission by the neutral gas transport Monte Carlo code EIRENE for each molecular fraction. Conversion factors (S/XB)$_{eff}$ were measured in various reference plasmas (T$_{e}$ = 22–30 eV, n$_{e}$ = 1–11 x 10$^{18}$ m$^{-3}$ and T$_{e}$ = 50–74 eV, n$_{e}$ = 1–5 x 10$^{18}$ m$^{-3}$) by desorbing prepared graphite samples which release a known amount of hydrogen with a known molecular distribution. LDS measurements were carried out in TEXTOR at hydrocarbon layers deposited on a graphite test limiter. The desorbed layers had been deposited within up to 3 minutes of plasma exposure, which is 40% of an ITER plasma discharge. The measured hydrogen inventories could be validated in post-mortem analyses. For the first time, LDS allowed for measuring the hydrogen inventory in the erosiondominated zone of a graphite limiter during a plasma discharge. The inventory was found to be larger during the plasma discharge than between two discharges. The deduced dynamic hydrogen inventory during the plasma discharge is 1–8 x 10$^{20}$ atoms/m$^{2}$, depending on the plasma conditions (H$^{+}$ flux density, plasma temperature). This is in line with a mean retention time of 1.9 to 3.6 ms. It was shown that LDS allows a sensitive measurement (detection limit below 10$^{20}$ m$^{-2}$) of hydrogen inventories in the surface of plasma-facing components during a plasma discharge.
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